Escherichia coli growth is quickly inhibited by picolinic acid or 1,10-phenanthroline. Growth resumes after a short adaptation period. Inhibition is overcome by Mn ions and can be attributed to an inhibition of RNA synthesis. Guanosine 5'-diphosphate 3'-diphophate increases after this treatment in some but not all Escherichia coli cell lines. Cultured mammalian NRK cells treated with picolinic acid or deprived of isoleucine are very responsive to prostaglandin E1. The activity of adenylate cyclase in membranes from these treated cells is increased. NRK cells treated with inhibitors of IMP dehydrogenase, to lower GTP levels, lose 50-80% of the response to isoproterenol or prostaglandin E1.